1. Field of the Invention
The present invention relates to a vaporizer for vaporizing a low temperature liquid such as liquefied natural gas (hereinafter referred to as LNG) by using a heat exchange with a heating medium.
2. Description of the Related Art
As a means for vaporizing a low temperature liquid such as LNG, a heat exchange between a low temperature liquid and a heating medium is generally used. For example, Japanese Patent sho 53-5207 discloses an intermediate medium type vaporizer that uses an intermediate medium in addition to the heat source fluid, vaporizing LNG by using heat exchange between the intermediate medium and the LNG.
FIG. 7 shows an example of such a heat exchanger. The diagram shows a LNG vaporizer, which comprises an intermediate medium evaporator E1, a LNG evaporator E2, and a natural gas (hereinafter referred to as NG) heater E3. Further, as a path for the heat source fluid (which is sea water in the example shown), there are sequentially arranged an inlet chamber 10, a large number of heat source tubes 12, an intermediate chamber 14, a large number of heat source tubes 16, and an outlet chamber 18, the heat source tubes 12 and the heat source tubes 16 being provided in the NG heater E3 and the intermediate medium evaporator E1, respectively. In the intermediate medium evaporator E1, there is accommodated an intermediate medium (such as propane) 17 whose boiling point is lower than that of sea water, which is the heat source fluid.
As shown in FIG. 8, the LNG evaporator E2 comprises a capsule-shaped shell 21, the closed end portion of which is separated from the other portion by a tube plate 25. Further, a horizontal partition 20 is secured in the closed end portion, whereby there are defined an inlet chamber 22 and an outlet chamber 24, which are separated from each other, the chambers 22 and 24 communicating with a large number of substantially U-shaped heat transfer tubes 23. The intermediate portion of each heat transfer tube 23 protrudes in the upper portion of the intermediate medium evaporator E1, and the end portions thereof pass through the tube plate 25 and secured thereto.
In the inlet chamber 22, there is provided an LNG supply portion 28 for introducing LNG, the LNG supply portion 28 being connected to an LNG supply source through a supply passage (not shown). In the outlet chamber 24, there is provided an NG discharge means 29, which is connected to the interior of the NG heater E3 through an NG duct 26.
In this vaporizer, sea water, which is the heat source fluid, passes the inlet chamber 10, the heat source tubes 12, the intermediate chamber 14, and the heat source tubes 16 before it reaches the outlet chamber 18. Heat exchange is performed between the sea water passing through the heat source tubes 16 and the liquid intermediate medium 17 in the intermediate medium evaporator E1 to vaporize the intermediate medium 17.
LNG, which is the object of vaporization, is introduced into the heat transfer tubes 23 from the inlet chamber 22. Through heat exchange between the LNG in the heat transfer tubes 23 and the evaporation intermediate medium 17 in the intermediate medium evaporator E1, the intermediate medium condenses, the heat of condensation vaporize the LNG and consequently NG is obtained. This NG is introduced into the NG heating chamber E3 from the outlet chamber 24 through the NG duct 26, and is further heated by heat exchange with the sea water flowing through the tubes 12 in the NG heating chamber E3 and then supplied to the place where it is required.
In the above LNG vaporizer (and other low temperature liquid heating heat exchangers of various types), a large thermal stress is generated when the low temperature liquid is abruptly introduced at a great flow rate at the time of starting. In view of this, at the time of speaking, as shown in FIG. 8, the supply flow rate is reduced to perform slow cooling operation, in which LNG is supplied little by little from the LNG supply portion 28 to the inlet chamber 22.
However, when the flow rate is thus reduced and LNG is caused to flow out little by little from the LNG supply portion 28, the LNG first flows down to the bottom portion of the shell 21, and then spreads over the entire inlet chamber 22, so that the bottom portion of the inlet chamber 22 is locally cooled prior to the other portions. For example, in the structure shown in FIG. 8, a marked temperature gradient as shown in FIG. 9 is generated during slow cooling, and thermal stress attributable to this temperature gradient is generated.
That is, it is difficult to effectively mitigate the thermal stress generated in the inlet chamber 22 solely by reducing the LNG supply flow rate as in the prior art. In particular, in a heat exchanger, which is frequently started/stopped, there is a fear of fatigue failure being generated in, for example, the welding portion between the shell 21 and the tube plate 25 or the welding portion between the tube plate 25 and the partition 20. Further, a similar temperature gradient is liable to be generated not only at the time of starting but when the LNG flow rate is reduce to maintain slow cooling at the time of temporary interruption of the operation of the heat exchanger.
The present invention has been made in view of the above problems. It is an object of the present invention to provide a vaporizer for vaporizing a low temperature liquid in which it is possible to effectively restrain the generation of thermal stress when effecting slow cooling at the time of starting, etc.
To achieve the above object, there is provided in accordance with the present invention a method for effecting slow cooling in a heat exchanger for heating a low temperature liquid which is equipped with an inlet chamber in which the low temperature liquid is introduced, wherein, when effecting slow cooling, the low temperature liquid is sprinkled in the inlet chamber at a flow rate lower than that at the time of normal operation.
In this method, the low temperature liquid is diffused and supplied to a wide region in the inlet chamber at a flow rate lower than that at the time of normal operation, so that the temperature gradient generated in the inlet chamber is reduced, thereby effectively mitigating the thermal stress.
More specifically, the inlet chamber is equipped with a normal operation supply means and a slow cooling supply means with a sprinkling function; during normal operation, the low temperature liquid is supplied at least from the normal operation supply means to the inlet chamber, and, during slow cooling, the low temperature liquid is supplied solely from the sprinkling means, whereby it is possible to supply a low temperature liquid suitable for slow cooling to the inlet chamber through the dedicated slow cooling supply means at the time of slow cooling, and, after the completion of the slow cooling, it is possible to supply an LNG suitable for normal operation by the normal operation supply means.
Further, in accordance with the present invention, the above methods is performed by a vaporizer comprising an inlet chamber, a heat transfer tube into which the low temperature liquid is introduced from said inlet chamber and in which the low temperature liquid is vaporized, and means for sprinkling the low temperature liquid in said inlet chamber.
As the means for sprinkling, various types can be adopted. For example, by constructing the means for sprinkling such that the low temperature liquid is sprinkled from a plurality of places in the inlet chamber, it is possible to further widen the sprinkling region than in the case in which the liquid is sprinkled from a single place.
Further, by installing the means for sprinkling such that at least a part of the upper half of the inner wall of the inlet chamber is included in the sprinkling region, the low temperature liquid gradually flows down after being sprinkled against the upper half of the inner wall, so that it is possible to spread the low temperature liquid more uniformly.
Further, by installing the means for sprinkling such that the welding portion in the inlet chamber is included in the sprinkling region, it is possible to simultaneously cool a plurality of members on either side of the welding portion, so that the difference in temperature between these members is reduced, whereby it is possible to more effectively prevent breakage due to the thermal stress at the welding portion attributable to the difference in temperature.
In this device also, it is more desirable to provide the inlet chamber with the means for sprinkling and a normal operation supply means for supplying the low temperature liquid at a higher flow rate than the means for sprinkling.
In that case, by providing a supply passage branching off from a common low temperature liquid supply source to the normal operation supply means and to the means for sprinkling, and by providing in the supply passage leading to the means for sprinkling a flow rate varying means for varying the supply flow rate independently of the supply passage leading to the normal operation supply means, it is possible to free adjust the low temperature liquid supply amount during slow cooling according to the situation.
The flow rate varying means may be a remote control valve which varies the flow rate of the low temperature liquid through manual remote control, or a temperature adjusting valve which adjusts the flow rate of the low temperature liquid so as to maintain the temperature in the inlet chamber at a preset target temperature. In the latter case, it is possible to automatically perform an operation for maintaining the temperature in the inlet chamber at a predetermined temperature, for example, during temporary interruption of the operation of the heat exchanger (so-called cool down maintaining operation).
In the present invention, there is no particular restriction regarding the concrete structure of the entire heat exchanger. However, in a structure in which the inlet chamber is adjacent to the outlet chamber for the low temperature liquid evaporation kuro through the intermediation of a partition, the partition is heated by the heated fluid passing the outlet chamber, and the difference in temperature between the partition and the other members constituting the inlet chamber tends to increase, so that the application of the present invention to the heat exchanger is particularly effective.